Well…
The .dmg suffix is mac-specific and has a number of implications. It can be compressed – or not, it can be in .iso, .hfs or hfs+ format or even fat, so it’s a bit like quicktime: It’s a container.
To make a bootable image file for a Mac from a PC, I’d choose to make the more universal .iso format.
Among others, can Nero create an .iso.
How you would go about to accomplish the “bootable Mac image” from a PC is another story.
Gunleik